I assume we're not talking about the actual Vox labeled CT3757, right?
We're talking the standard Celestion-labeled G12 alnico? T731? T650?
The celestion-labeled T731 was used in some of the Selmer amps, so there would be that market for them. I'm not sure what amps used the T650 (or any of the other G12 alnicos variants). Maybe some Marshalls, but those were labeled Marshall, I think. So, unless they're labeled Vox or Marshall, the value would likely be more as a great speaker rather than a restoration item that people would pay through the nose for. Don't ask me how much I paid for a matched pair of Vox CT3757's. It was obscene.
In really great condition, I'd guess around $300 per speaker, maybe a little more for the T731 because you might get a Selmer owner desparate for it.
"Fights begin, finger prints are took, days is lost, bail is made, court dates are ignored, cycle is repeated." - Georgia's Favorite Son, Earlie Cuyler